Once you create 8 mono audio tracks, follow these steps:

1. Make sure that the input routing of each track is set to the appropriate input bus (Left 1, Right 2, Left 3, etc.)

2. Make sure that the record enable button is selected on each audio track. - at this point you should see the meters moving on the actual audio tracks.

3. Once you see the audio meters moving on the audio tracks, you can select the Record button on the transport bar and audio will be recorded into Cubase.

Hope this helps!